Exploring the environment – Cont’d
“If you can’t clean your surroundings, then don’t make it dirty.”
– The Fresh Quotes
HELLO, STUDENTS. How are you this week? Are you ready for another biology lesson? Well, I am, so let us get cracking! I hope you have found our lessons on the environment interesting and that they have been prompting you to go out and explore while, at the same time, causing you to develop an appreciation for the world around you, its intricacies and its beauty. This week, we will be looking at how the different elements interact together to create a unit.
LIVING ORGANISMS IN THE ENVIRONMENT
Living organisms depend on each other for survival. In order to survive, organisms need energy. This energy is released from food, but organisms do not all get their food in the same way. Plants produce their own food using the sun’s energy, while animals obtain food that is already manufactured. Plants are known as producers because they have the ability to produce their own food, and animals are known as consumers because they feed on already manufactured food. Consumers can feed directly or indirectly on the food produced by plants. Those animals that feed directly on plants are known as herbivores or first-order/primary consumers. All other animals are carnivores, feeding on the animals only, or omnivores, feeding on both plants and animals. Those that feed directly on the herbivores are known as second-order/secondary consumers, e.g., praying mantis. Those that feed on the animals that have eaten animals are known as thirdorder/tertiary consumers, e.g., lizards.
FEEDING RELATIONSHIPS
The feeding relationships of animals follow a particular order. It is important for you to remember that this is so. This relationship forms what is known as a food chain. Food chains occur wherever plants and animals are found, i.e., both on land – these are terrestrial (arboreal/trees and edaphic/soil), and in water (fresh water and marine) – these are aquatic.
DEFINITION OF FOOD CHAIN
A food chain is the sequence of organisms through which energy passes, after entering the ecosystem, until it reaches the highest trophic level.
In simple terms, the food chain shows what eats what in a community.
I have just introduced a new concept, did you notice? The trophic level; what is this?
Trophic level
This is the level of the food chain at which an organism feeds.
The plant forms the first trophic level.
EXAMPLES OF FOOD CHAINS TERRESTRIAL
Leaf – aphid – praying mantis – lizard Grass – cricket – toad – snake Gras – grasshopper – insect-eating bird – hawk Algae – snail – small fish – large fish Pondweed – mosquito larva – small fish – large fish
Water weed – tadpole – guppy – large fish
Notice the following important points about food chains:
The food chains all begin with a plant. This has to be so because the plants are the producers. Only plants are capable of trapping the energy from the sun and converting it to chemical energy stored in food.
The producer can be the entire plant or parts of the plant, such as seeds, flowers or even leaves which can be dead or decaying.
The arrows point in the direction in which the energy flows, i.e., all arrows point from the producer towards the consumer.
These food chains all have four trophic levels but there are times when food chains have only three levels; they rarely have more than four. Why is this so? As energy flows from the producer to the first-order/primary consumer, up to 90% of it is lost, so only 10% is transferred to the primary consumer. Some secondary consumers are able to transfer 20% of the energy consumed to the next level. This is because the digestion of proteins is more efficient than the digestion of carbohydrates, and animals have a lot more proteins than plants.
HOW IS THE ENERGY LOST?
Energy is lost as it flows from one trophic level to the next (see below):
As heat from the respiration of both plants and animals.
Remains unused when parts of plants and animals remain uneaten.
As urine in the excretion of animals. As indigestible parts, e.g., nails, bones, teeth, hair, cellulose fibres, which are passed out in faeces.
This means that less energy is available at each trophic level in the food chain and, hence, only fewer organisms can exist at the next level. As the organisms consume the available energy at a particular trophic level, most of it is used and only 10% or 20% is passed on, hence the next level will only be able to accommodate fewer organisms. This means that there is rarely enough energy to support more than four levels.
Do you realise that if we were all to become vegetarians, more people could be fed? This would cut down on the number of links in the food chain and, in essence, the amount of energy that is lost at the different trophic levels.
You would appreciate that there is no community that is so simple that only one type of organism feeds on any plant or is the food for one type of animal. Therefore, single food chains cannot be used to describe the feeding relationships in a community. Many interconnected food chains are found in a habitat. These are best described using not food chains, but what are known as food webs.
HOW DO WE DEFINE A FOOD WEB?
A food web shows the feeding (energy) relationships between all the organisms in a community. It is made up of many interlinked food chains.
Next week, we will look more closely at the food web. Until then, have a good week!